1. Field of the Invention The present invention relates to improvements in poly(vinyl chloride) compositions which exhibit improved adhesivity to fire retardant polyamide compositions which find particular utility in the construction of multi-layer structures, particularly for the fabrication of improved structures for use in forming electrical insulation structures for wires and cables.
2. Description of the Prior Art
Multi-layer structures typically comprise at least two or more layers of materials featuring different physical properties, which properties are optimized in order to satisfy a particular need at hand. The goal of the formation of such structures is to provide a structure which is directed to a particular need, and wherein the individual layers of the structures act to form a composite, and where each layer of material contributes to serve the function for which the structure is designed. Examples of such structures are well known to the art, and include composites which are used to form films, wherein each layer forming the film may feature a particular property, such as impermeability to a gas, or as a blocking layer to light or radio energy of a particular wavelength. Another example of such a structure may be those which would be used to form an insulation structure for an electrical conductor, where each layer forming a jacket about the conductor might feature improved abrasion resistance, or improved dielectric strength, or the like. Other uses of such structures are notorious and well known to the art.
The use of such multi-layer structures forms the state of the art in electrical conductors. These conductors typically consist of a wire conductor (which is frequently a metal, including copper) which is surrounded by a multi-layer structure. Examples of such constructions include those taught in U.S. Pat. Nos. 3,576,940; 3,860,686; 4,079,191; 4,292,463; 4,327,248; 4,419,538; 4,472,597; 4,510,348; 4,626,619; and U.S. Pat. No. 4,691,082. Therein are disclosed a variety of constructions wherein each of the layers may be a polymer, a metal, an elastomeric material, a fibrous material, etc., where each layer offers a particular advantage, i.e., dielectric strength, water impermeability, etc.
Two favorable materials which may be used in the construction of electrical conductors are poly(vinyl chloride) (hereinafter interchangeably referred to as "PVC") and polyamides (hereinafter interchangeably referred to as "PA"). These materials are favored as the PVC generally forms a good flexible insulator, while simultaneously, PA generally exhibits good abrasion resistance properties. Examples of electrical conductors having a multi-layer structure of an inner layer of poly(vinyl chloride) and an outer layer of polyamide are taught in Japanese Patent 59146105, British Patent 1257810, and Dutch Patent 6917475, and U.S. Pat. No. 4,868,054.
While such multi-layer structures comprising a layer of PVC and PA when used as electrical conductors are seen to perform well, such a structure retains drawbacks. One problem is known to exist if the electrical conductor is stored outside and exposed to rain and high humidity. Upon contact with water, the polyamide layer tends to absorb moisture and expand considerably. This expansion causes the polyamide layer to swell away from the PVC layer and wrinkle. When the electrical conductor is then fed through a conduit, i.e. typically during installation, the wrinkled polyamide layer tears and the electrical conductor becomes unacceptable for use for its intended purpose. Further, a condition known to the art as "horse collaring" is known to occur, wherein sharp flexure of the wire causes separation of the polyamide and the poly(vinyl chloride) layers and the formation of bulges in the exterior layer in a direction transverse to that of the wire's length. The wire or cable exhibits an increased tendency to tear at such bulges, especially as the wire is subjected to mechanical stress as might be occasioned as the wire is pulled through a conduit, during installation of said wire or afterwards, due to vibration, thermal effects, etc.
It would be desirable to have a multi-layer structure where good adhesion exists between the PVC layer and the polyamide layer, and which exhibits good fire retardant properties. Fire retardancy is a highly desirable feature, particularly where a multi-layer structure is used to form electrical insulation layers enrobing an electrical current conductor, such as a wire or plurality of wires. As is known, electrical cables present in a commercial, residential or other structure (i.e., office buildings, homes, public buildings, and facilities) comprise a plurality of individual wires which are contained within a jacketing layer, or other plenum. In the event of a fire within such a structure, the hazard of such a fire would be greatly compounded where the insulation layer enrobing an electrical current conducting wire were to burn or melt off, and thereby expose the wire to contact with an alternate current path. Such a current path could be the structure or element of a structure within which the wire is present, or in the more undesirable alternative, with a person's body. Further, melting of an insulating layer of a current conducting wire, particularly where proximately located to other current conducting wires with similarly degraded insulation layers raises the risk of contact between the wires and the formation of a short circuit; such an event raises the risk of ignition of surrounding materials. Alternately, if surrounding materials are not ignited, there remains the risk and consequent hazard of the ignition, burning or melting of the materials used to form insulating layers over the wire. Such materials frequently emit harmful emissions, which in sufficient concentration may prove fatal. Accordingly, there exists a continuing need in the art in providing multi-layer structures which feature fire retardancy, and good physical characteristics.
Multi-layer structures having a PVC layer, an adhesive layer, and a polyamide layer are known. Japanese Patent 62041039 teaches an adhesive layer comprising an acid-modified olefinic polymer and an acid-modified halogen-containing olefinic polymer. German Patent 1669973 teaches an adhesive layer having: (1) a primer layer comprising a mixture of epoxide compounds, hardener, and solvent, and (2) an epoxy adhesive layer. Further, U.S. Pat. No. 4,868,054 to Kartheiser describes a multilayer structure comprising a first layer comprising poly(vinyl chloride) and a second layer comprising a polyamide; the structures feature good interfacial adhesion between the layers.
While these multi-layer structures known to art may be beneficially used, there remains a continuing need in the art for multi-layer structures which would be particularly useful as insulation structures for electrical conductors wherein improved adhesion between the layers exists, and which feature a high degree of fire retardancy.